The invention relates to superconductors and methods of manufacture thereof.
By superconducting member is meant a member which will exhibit superconductivity when its temperature is lowered below its critical temperature. Materials of particular interest in this field are those which have comparatively high critical temperatures and comparatively high critical magnetic fields. Such materials are compounds of the A15 crystal structure having the general formula A.sub.3 B where A comprises niobium or vanadium and B typically comprises one or more of the elements aluminium, gallium, indium, silicon, germanium, and tin.
The invention is more particularly concerned with the manufacture of a superconducting member comprising a large number of fine superconductive filaments supported in an electrically conductive, non-superconductive matrix, and is an improvement in or development of the inventions described in British Pat. Nos. 1,333,554 and 1,394,724.
It is desirable to have regions of pure metal with high electrical and thermal conductivity, for example pure copper or aluminium, incorporated in multifilamentary superconducting composites to provide additional stabilization. The pure metal provides dynamic stabilization by damping flux movements and by acting as a sink for any heat produced. To be effective the pure metal should be situated as close to the superconducting filaments as possible.
Provision of a pure metal such as copper adjacent to the filaments presents little difficulty with a ductile superconductor like the niobium-titanium alloys because there is little pick up of niobium or titanium by copper under the annealing conditions carried out for fabrication or heat treatment to obtain optimum critical currents in the niobium-titanium superconductor.
A.sub.3 B compounds with the A15 crystal structure are sometimes superconducting with high temperatures of transition from the superconducting to the normal state. These compounds cannot be produced as multifilamentary superconductors by techniques suitable for ductile superconductors because they are very hard and brittle materials. One method suitable for producing A.sub.3 B compounds as multifilamentary superconductors is described in British Patent No. 1,333,554. For example, rods or wires of the element A are embedded in a matrix of a carrier metal containing the element B. Copper is a suitable carrier metal for the production of Nb.sub.3 Sn or V.sub.3 Ga by this method and the alloy of the carrier metal with the element B is conveniently referred to as "the bronze" or "the Cu-B alloy". The B element is generally in solid solution in the carrier metal since this provides a ductile alloy, but the method can also work when the B element is also present in other phases in the bronze. The element A and the Cu-B alloy may both contain additions of other elements. The composite of rods of element A in a matrix of bronze is fabricated by a simple mechanical deformation process to produce fine filaments of element A in the required configuration in the bronze matrix. The filaments of element A are then converted to compound A.sub.3 B by reaction with the B element from the bronze by heating in a temperature range in which the bronze in contact with element A remains in the solid state.
The compound A.sub.3 B produced by this technique will be left in a copper alloy still containing some element B in solid solution. The element B increases the resistivity of the copper considerably so that the residual bronze matrix containing the A.sub.3 B filaments is not the best material to provide dynamic stabilization. If the extra stabilization provided by pure metals such as copper or aluminium is required these materials must be incorporated into the composite superconductor in some way. In some circumstances where the superconductor can be handled in the reacted condition (i.e. with the A.sub.3 B filaments present) it may be possible to put the pure metal with suitably low resistivity on the outside of the superconductor by some room temperature operation, e.g. by electrodeposition. However, in many circumstances one cannot avoid heating the pure metal with the composite superconductor. For example, this would occur (a) if the pure metal is put on to the composite by some hot working process, or (b) if the pure metal is incorporated inside the composite when it must of necessity be present during some of the fabrication processes, or (c) if the superconductor is wound in unreacted form to the required final shape (e.g. solenoid winding) and then reacted. In all these examples the pure metal will be in contact with the bronze and on heat treatment the element B will diffuse from the bronze into the pure metal. This will increase the resistivity of the pure metal and make it less effective for stabilization.
A solution to this problem, allowing pure metal to be incorporated in the superconducting composite, is to isolate the pure metal by a barrier of metal which prevents diffusion of the B element from the bronze into the pure metal. Besides being impermeable to the B element at the reaction temperature, the barrier material must be insoluble in the pure metal and must be ductile enough to be fabricated with the superconductor and provide a continuous barrier separating the pure metal from the bronze after these treatments. Use of such barrier materials is described in Patent No. 1,394,724. In the example in which Nb.sub.3 Sn was produced by reaction of niobium with a copper tin bronze and where copper was used for stabilization the preferred barrier material was tantalum. In producing superconducting composites it has been found that although tantalum is a satisfactory barrier material when the deformation is not too great, failures of the tantalum barriers have occurred after extensive deformation for producing multifilamentary wire with the particularly fine filaments desirable for optimum superconducting properties. In experiments under these conditions, we have observed the niobium filaments have remained continuous and deformed in a fairly uniform manner. We concluded the mechanical fabrication of the superconducting composites would be easier if the filaments and barriers had the same mechanical properties, specifically if the same material were used for both.
The use of barriers of the A metal is described in British Pat. No. 1,394,724. It is pointed out that the A metal is often impermeable to the B element at the temperature at which the A.sub.3 B compound is formed by solid state reaction. In particular niobium dissolves very little tin in solid solution below about 900.degree. C. The use of thin tubes of metal A (e.g. niobium) filled with bronze and embedded in a matrix of pure metal is described for the special case where the diameter of the tubes is in the range necessary for filamentary stabilization. However in more general applications, the formation of a layer of A.sub.3 B at the interface between the barrier of the element A and the bronze may take these barriers less suitable than barriers of other materials which do not react with the bronze to form high field superconducting compounds. Thus the layer of A.sub.3 B compound at the barrier may shield the pure metal from flux jumps outside the barrier so that the pure metal is prevented from providing effective stabilization. Also the layer of A.sub.3 B compound at the barrier may itself cause instabilities since it would generally present a larger dimension transverse to the magnetic field than the A.sub.3 B compound filaments would. On the other hand the close proximity of the pure metal to these A.sub.3 B compound layers would tend to stabilise them against flux jumping. Apart from these objections to the A.sub.3 B formation on stability grounds its formation can lead to the failure of the barrier during heat treatment. The barriers do not thin down completely uniformly on deformation. At the thinner parts A.sub.3 B compound formation can penetrate through the barrier even although the majority of the barrier remains effective. Once A.sub.3 B compound is in contact with the pure metal the latter can start picking up B atoms and its electrical resistivity will start to rise.